A better fix for bug#21739

A better fix for bug#21739

* src/buffer.c (set_update_modelines_for_buf): New function.
(Fkill_buffer): Use it to set the global variable
update_mode_lines if the killed buffer was displayed in some
window.  Don't set windows_or_buffers_changed.  This is a better
fix for bug#21739 than the previous fix, since it will cause only
redisplay of mode lines, not of entire windows, but will still
catch attention of x_consider_frame_title in xdisp.c, which
redraws the frame title.